The iPhone has consistently topped itself each year, posting
record-breaking sales weekends for each of the past four major
launches. As for this year,
which should see Apple debut at least one new large-screened
iPhone, RBC Capital
Markets analyst Amit Daryanani says (via
Electronista) Apple could easily sell 10 million iPhone 6
units in its debut weekend alone.

Last year's iPhone launch saw 9 million units sold in the opening
weekend, but Apple had two different iPhone models to sell last
year as opposed to one. With Apple’s
stock reaching an all-time high this week, it seems
reasonable to predict 10 million iPhone 6 units sold in the
opening weekend, but that's more likely to happen if Apple
releases two different iPhone models like it did last year. Apple
is expected to debut a 4.7-inch iPhone 6; there have also been
rumors of a larger 5.5-inch iPhone, but various reports have said
that phone's launch
may be delayed due to production issues.